In the manufacture of tobacco products, tobacco by-products such as tobacco stems, leaf scraps, and tobacco dust produced during the manufacturing process can be recycled to reclaim their useful tobacco content. In the past, such tobacco by-products have been formed into what is known in the industry as reconstituted tobacco sheets.
Commonly owned U.S. Pat. No. 5,203,354 discloses a process for manufacturing reconstituted tobacco sheet by casting a tobacco containing slurry containing 20% solids and 80% liquid onto a rotating cylindrical dryer which is heated by gas, electricity or steam and the dryer reduces the moisture content of the slurry to 12-40%. In order to avoid deterioration of flavor and other properties of the tobacco the dryer surface temperature is from 180 to 200.degree. F. A process for making reconstituted tobacco sheet by casting or extruding a tobacco slurry at 80-200.degree. F. onto a continuous stainless steel belt and drying the slurry at 200-700.degree. F. is described in commonly owned U.S. Pat. No. 5,724,998. Commonly owned U.S. Pat. No. 5,584,306 discloses a process for making reconstituted tobacco sheet by reverse-roll coating a slurry at 120-160.degree. F. onto a stainless steel belt traveling at a line speed of 20-400 feet/minute and drying the slurry in a primary dryer and secondary dryer after which the sheet is doctored from the belt. In the examples of the '306 patent the belt speed was 30 fpm, the dryer was a gas-fired, air impingement dryer, and the sheet emerging from the secondary dryer had a moisture content of approximately 15% and sheet weight of 11.6-14.6 g/ft.sup.2.
U.S. Pat. No. 5,099,864 describes a process for making reconstituted tobacco sheet wherein a tobacco slurry at 140-180.degree. F. and solids content of 14-20% is cast as a 15-40 mil thick sheet on a stainless steel belt moving at 1.5 m/min and dried to a moisture content of about 14% and weight of 75-150 g/m.sup.2. U.S. Pat. Nos. 4,306,358 and 5,743,022 describe apparatus for the drying of a reconstituted tobacco slurry.
During the drying process, prior art processes have utilized air impingement dryers and other drying equipment to remove the excess moisture. In order to effect rapid removal of water from the slurry, gas-fired air impingement dryers have been placed both above and below the conveyor belt downstream of the casting device. In such gas-fired, air impingement dryers, the thin layer of slurry arrives in a relatively cool state due to heat transferred to the steel belt and during heating by the dryers, a skin or "film" can form on the surface of the slurry which lowers the rate at which moisture can be evaporated from the slurry.
In view of the state of the art of making reconstituted tobacco sheet, there is a need for a processing apparatus and method which overcomes the slow evaporation problem caused by prior art techniques for drying reconstituted tobacco slurries. It would be desirable if such apparatus could be incorporated in existing processing equipment.